Strip planarity measurement is known, for example, from EP 1 369 186 A2 (U.S. Pat. No. 6,853,927) that uses a surface evenness measuring roller with at least one integrated beam sensor. The beam sensor extends longitudinally in a wavy or straight line over the length of the roller for the purpose of determining tensile forces acting on said roller from the strip. The strip is partially wrapped around the sensor roller wherein during the determining of the tensile force the beam sensor with its starting region and its end region is at least occasionally within an arc of contact that is formed by the band. The sensor beam is supported at its end on load cells and/or force transmitters.
The beam sensor is fastened to the roller body with the help of two preloading bolts which preloading bolts are guided through internal bore holes of the force transmitters and apply a predetermined preloading force to the force transmitters. Such measures are also known in the realizing of radially stressed and annulus-shaped covers for the force transmitters (see also DE 42 36 657 C2).
The force transmitters are piezoelectric units, usually of quartz (piezo quartzes) which require a bias voltage in order to be able to measure pressure forces. However the piezo quartz piercing preloading bolts necessarily produce a force shunt such that all the measured forces are not introduced to the force transmitters and rather a portion thereof is introduced into the preloading bolts. The force shunt can amount to up to 30%. Although the influence of the force shunt can be initially filtered out by means of a calibration, this is effective only where the force shunt remains constant. In fact however there is a danger that the magnitude of the force shunt will change in operation, e.g. due to temperature changes, and thereby falsify the measurement result. In addition the measured signals will not then correspond to the full measured force. One might well consider using as force transmitters strain gauges which require no bias voltage whereby no force shunt is generated. Such strain gauges however have in comparison with piezo quartzes a relatively low measuring accuracy.